Conventionally, an image forming apparatus, such as a copier, a printer, and a facsimile, has a fixing device which fixes a toner of an unfixed toner image formed on a transfer material. A conventional fixing device includes, for example, (i) a fixing roller whose surface is heated, (ii) a pressure roller which contacts the fixing roller, and (iii) surface temperature detecting means for detecting a surface temperature of the fixing roller.
In an image forming apparatus including this fixing device, when a main switch is turned ON, a warm-up for warming the fixing device is carried out. However, the temperature of the fixing device is low in the case in which the main switch has been in an OFF state for a long time, such as in the case in which the main switch is turned ON first thing in the morning. On this account, even if the main switch is turned ON for copying, etc. after the main switch has been in the OFF state for a long time, the conventional warm-up may not warm the fixing device sufficiently.
When a printing is carried out even though the fixing device is not warmed sufficiently, heat of the fixing roller of the fixing device is extracted by the transfer material. In this case, the surface temperature of the fixing roller decreases drastically, so that a sufficient fixation performance cannot be obtained. Therefore, the conventional image forming apparatus requires a wait time for warming the fixing device sufficiently. On this account, proposed is a fixing device which does not require a longer warm-up time and can prevent the surface temperature of the fixing roller from decreasing (for example, see Documents 1 and 2).
Document 1 discloses a temperature control device of a heating roller fixing apparatus which includes a fixing roller and a pressure roller each of which has a heater therein, and the temperature control device switches a set temperature of the heater of the pressure roller between at the time of the warm-up and after the warm-up.
In Document 1, the temperature of the pressure roller is set to a high set temperature from the start of the warm-up till the end of it, and the temperature of the pressure roller is switched to a low set temperature after the warm-up. Thus, it is possible to narrow a temperature control range.
Moreover, Document 2 discloses a fixing device in which (i) after the warm-up, a surface temperature of a fixing roller is controlled so as to be temporarily a high set temperature that is higher than a set temperature of the surface temperature of the fixing roller, and (ii) at least one of the temperature level and time length of the high set temperature is variable.
As described above, in Document 2, even after the warm-up, the surface temperature of the fixing roller is controlled so as to be temporarily the high set temperature. On this account, even if copying is consecutively carried out after the warm-up, the surface temperature of the fixing roller just decreases to the set temperature. Moreover, in addition to this, at least one of the temperature level and time length of the high set temperature is changed depending on the surface temperature of the fixing roller at the start of the warm-up. On this account, since the temperature level or the time length of the high set temperature is controlled depending on how much heat the fixing roller is accumulating, it is possible to prevent power from being consumed wastefully.
Document 1
Japanese Unexamined Patent Publication No. 28177/1984 (Tokukaisho 59-28177 (published on Feb. 14, 1984))
Document 2
Japanese Unexamined Patent Publication No. 127565/1993 (Tokukaihei 5-127565 (published on May 25, 1993))
In Document 1, since the temperature of the pressure roller is the high set temperature from the start of the warm-up till the end of it, the increase in the warm-up time is prevented. However, since the temperature of the pressure roller is switched to the low set temperature after the warm-up, the surface of the fixing roller is not heated sufficiently.
That is, Document 1 is just to narrow the temperature control range from the surface temperature of the fixing roller after the warm-up and the surface temperature of the fixing roller after the fixing roller is sufficiently rotated and waits ready. On this account, the warm-up terminates when the surface temperature of the fixing roller has reached the set temperature. If the printing is started in this state, the surface temperature of the fixing roller decreases, so that the sufficient fixation performance cannot be obtained.
Further, in Document 1, the fixing roller is kept heated so that the surface temperature of the fixing roller is maintained to the set temperature even after the warm-up. Therefore, a problem in Document 1 is that the amount of power consumption is large.
In Document 2, even after carrying out copying consecutively, the surface temperature of the fixing roller just decreases to the set temperature. This is realized by heating the fixing roller after the warm-up so that the surface temperature of the fixing roller is temporarily the high set temperature. In this case, it is necessary to keep heating the fixing roller at high temperature not only at the time of the warm-up but also after the warm-up. Therefore, as with Document 1, a problem in Document 2 is that the amount of power consumption is large.
In Document 2, in order to prevent power from being consumed wastefully, at least one of the temperature level and time length of the high set temperature is changed depending on how much heat the fixing roller is accumulating. However, since the fixing roller is kept heated at high temperature after the warm-up also in this case, the amount of power consumption is large.
Here, in order to reduce the amount of power consumption, it may be possible to think of a driving method (hereinafter referred to as “energy saving mode”) for not heating the fixing roller except at the time of the warm-up and at the time of the printing. However, in the energy saving mode, even if the printing is requested immediately after the main switch of the image forming apparatus is turned ON, the surface temperature of the fixing roller is not high enough. Therefore, a problem here is that the wait time from when a command for starting printing is inputted until when the printing is started becomes long.
That is, when the main switch of the image forming apparatus is turned ON, cleaning of a photoreceptor is carried out in addition to the heating of the fixing roller, as the warm-up. In the case of the energy saving mode, the warm-up is carried out while the cleaning of the photoreceptor is being carried out, and the warm-up terminates when the cleaning of the photoreceptor terminates.
In this case, if the fixing roller is heated only while the cleaning of the photoreceptor is being carried out, the surface temperature of the fixing roller does not become high enough. That is, as shown in FIG. 9, the heating of the fixing roller terminates at a time point t13 at which the warm-up terminates. In this case, the heating of the fixing roller terminates before the surface temperature of the fixing roller reaches a printable temperature. Moreover, since the heating of the fixing roller terminates, the surface temperature of the fixing roller further decreases as time advances. Therefore, in order to carry out fixing securely, it is necessary to start the heating of the fixing roller at a time point t14 at which the command for starting the printing is inputted. In addition, this heating of the fixing roller needs to be carried out until the temperature of the fixing roller reaches the printable temperature. As a result, the wait time from when a command for starting printing is inputted until when the printing is started becomes long.
The present invention was made to solve the above-described problems, and an object of the present invention is to realize a fixing device and an image forming apparatus each of which is such that, even if the printing is started when the surface temperature of the fixing device is completely low, the amount of power consumption is small and the wait time from when a command for starting printing is inputted until when the printing is started is short.